Cellulose esters such as cellulose triacetate (CTA), cellulose acetate propionate (CAP), and cellulose acetate butyrate (CAB), are used in a wide variety of films in the liquid crystal display (LCD) industry. For example, they can be used as protective and compensator films in conjunction with polarizer sheets. These films are typically made by solvent casting, and then are laminated to either side of an oriented, iodinated polyvinyl alcohol (PVOH) polarizing film to protect the PVOH layer with regards to scratching and moisture ingress, while also increasing structural rigidity. Alternately, as is the case of compensator films, they can be laminated with the polarizer stack or otherwise included between the polarizer and liquid crystal layers. Cellulose esters have many performance advantages over other materials such as cyclolefins, polycarbonates, polyimides, etc. that are also used in display films. However, optical birefringence requirements currently often dictate that the latter be used instead.
Besides serving a protective role, these films can also improve the contrast ratio, wide viewing angle, and color shift performance of the LCD. For a typical set of crossed polarizers used in an LCD, there is significant light leakage along the diagonals (leading to poor contrast ratio), particularly as the viewing angle is increased. It is known that various combinations of optical films can be used to correct or “compensate” for this light leakage. These films must have certain well-defined birefringence (or retardation) values, which vary depending on the type of liquid crystal cell used because the liquid crystal cell itself will also impart a certain degree of undesirable optical retardation that must be corrected. Some of these compensator films are easier to make than others, providing manufacturing savings, so compromises are often made between performance and cost. Also, while most of the compensator and protective films are made by solvent casting, there is a push to make more films by melt extrusion so as to eliminate the need to handle environmentally unfriendly solvents. Having a material with more controllable optical retardation, that can be made by both solvent and melt casting, such as the material in the present invention, allows for greater flexibility in creating these films.
Compensator and optical films are commonly quantified in terms of birefringence, which is, in turn, related to the refractive index n. The refractive index is typically in the range of 1.4 to 1.8 for polymers in general, and approximately 1.46 to 1.50 for cellulose esters. The higher the refractive index, the slower the speed the light wave propagates through that given material.
For an unoriented isotropic material, the refractive index will be the same regardless of the polarization state of the entering light wave. As the material becomes oriented, or otherwise anisotropic, the refractive index becomes dependent on material direction. For purposes of the present invention, there are three refractive indices of importance denoted nx, ny, and nz, which correspond to the machine direction (MD), the transverse direction (TD), and the thickness direction respectively. As the material becomes more anisotropic (e.g. by stretching), the difference between any two refractive indices will increase. This difference in refractive index is referred to as the birefringence of the material for that particular combination of refractive indices. Because there are many combinations of material directions to choose from, there are correspondingly different values of birefringence. The two most common birefringence parameters are the planar birefringence (Δe) and the thickness birefringence (Δth), and are defined as:Δe=nx−ny  (1a)Δth=nz−(nx+ny)/2  (1b)
The birefringence Δe is a measure of the relative in-plane orientation between the MD and TD and is dimensionless. In contrast, Δth gives a measure of the orientation of the thickness direction, relative to the average planar orientation.
Another parameter often used to characterize optical films is the optical retardation R. R is simply the birefringence times the thickness (d) of the film in question. Thus,Re=Δed=(nx−ny)d  (2a)Rth=Δthd=[nz−(nx+ny)/2]d  (2b)
Retardation is a direct measure of the relative phase shift between the two orthogonal optical waves and is typically reported in units of nanometers (nm). Note that the definition of Rth varies with some authors, particularly with regards to the sign (+/−).
Materials are also known to vary with respect to their birefringence/retardation behavior. For example, most materials when stretched will exhibit a higher refractive index along the stretch direction and a lower refractive index perpendicular to the stretch. This follows because, on a molecular level, the refractive index is typically higher along the polymer chain's axis and lower perpendicular to the chain. These materials are commonly termed “positively birefringent” and represent most standard polymers, including commercial cellulose esters. In contrast, polymers that have a larger refractive index in the transverse direction (relative to the stretch direction) are termed “negative birefringent.” It is important to note that there could be some confusion in the use of the terms “positive” and “negative” birefringence between the fields of polymer science and optical physics. From a polymer science standpoint, positive and negative birefringence refer specifically to the refractive indices along the chain as described above (i.e., essentially a material property). In contrast, in optical physics and many LCD related discussions, positive and negative birefringence refer specifically to the overall film or plate's birefringence behavior. As will be described later, a positively birefringent polymer can be used to make either positive or negative birefringent films or “plates” simply by changing the film orientation. Thus it is important to understand the context of “birefringence” and whether it relates to material behavior or overall structure, when assessing the meaning of “positive” or “negative.” Unless otherwise stated, the terms Δe, Δth, Re, and Rth in this application, including the claims, are calculated as indicated in equations (1a), (1b), (2a), and (2b) above, respectively.
Another useful parameter is the “intrinsic birefringence,” which is a property of the material and is a measure of the birefringence that would occur if the material was fully stretched with all chains perfectly aligned in one direction. For purposes of the present invention, intrinsic birefringence provides a measure of the sensitivity of a given polymer to a given amount of chain orientation. For example, a sample with high intrinsic birefringence tends to exhibit more birefringence during film formation than a sample with low intrinsic birefringence, even though the relative stress levels in the film are the same. Throughout this application, unless otherwise noted, we will refer to positive and negative intrinsic birefringence to clearly denote material behavior as opposed to the overall film or plate birefringence.
One additional parameter useful in the assessment of the overall optical properties of a film is the stress optical coefficient (SOC). The SOC gives the amount of birefringence that forms for a given amount of stress applied to the film. It is the stress that causes the chains to orient and thus be birefringent. Note that there are two distinct values of the SOC depending to whether the material is above or below the glass transition temperature Tg. If below Tg, the birefringence that forms is due to van der Waal bond deformation and is referred to as the “glassy” SOC. In the case of melt extrusion, the more useful parameter is the birefringence that forms from stress applied above Tg (i.e. the “rubbery” SOC) as this causes the chain alignment and birefringence more commonly associated with flow. Unless otherwise noted, “SOC” in this application refers to the rubber SOC value. The SOC is typically measured by applying a given stress to a film at a temperature above Tg, and then measuring the resulting birefringence. This is a parameter that is sensitive to the chain chemistry of the material.
To understand birefringence and SOC, it helps to visualize what happened on a molecular level. A propagating light wave “slows down” (i.e. the refractive index increases) every time it interacts with electrons in a material. For a given atomic bond (e.g. a carbon-carbon or carbon-oxygen bond), this interaction will be greater if the light wave is polarized along the bond direction and weaker if aligned perpendicular to it. Thus, the atomic level refractive index or “bond polarizability” is higher along a bond direction as opposed to perpendicular to the bond. To obtain the macroscopic refractive index, one must average over all of the bonds and bond angles in the material. If, on average, the bonds tend to align more preferentially in one direction than another, then the material will be birefringent. In contrast, if the bond angles are randomly distributed in all directions, as for example with an unoriented material, then the refractive index will be constant in all directions and the birefringence will be zero.
As a polymer is stretched (for example, in the x direction), the polymer chains will align in the stretch direction. For most (but not all) materials, this means that more atomic bonds are also going to be preferentially aligned in the stretch direction. Consequently, the refractive index nx will be greater than ny and nz and they are referred to as “positively birefringent”. In other words, the refractive index for a positive birefringent material is highest in the orientation direction. For these materials, the SOC is positive.
In contrast there are some materials that are negatively birefringent and have a negative SOC. For these materials, there are more atomic bonds aligned perpendicular to the chain axis rather than parallel. Thus, when the material is stretched in the x direction, there is a preferential alignment of atomic bonds in the y and z directions. As a result, the refractive index refractive index nx will be less than ny and nz after stretching. Examples of this type of negatively birefringent material include PMMA and polystyrene.
Negative birefringent polymers exhibit a higher refractive index perpendicular to the stretch direction (relative to the parallel direction), and consequently also have a negative intrinsic birefringence. Certain styrenics and acrylics are known to have negative birefringent behavior due to their rather bulky side groups. Zero birefringence, in contrast, is a special case and represents materials that show no birefringence with stretching and thus have a zero intrinsic birefringence. Such materials are ideal for optical applications as they can be molded, stretched, or otherwise stressed during processing without showing optical retardation or distortion. Such materials are extremely rare.
The actual compensation films that are used in an LCD can take on a variety of forms. These include biaxial films, where all three refractive indices differ and two optical axes exist, and uniaxial films, where two of the three refractive indices are the same and have only one optical axis. There are also other classes of compensation films, where the optical axes twist or tilt through the thickness of the film (e.g. dischotic films). In general, the type of compensator film that can be made from a given material is a function of the intrinsic birefringence characteristics of the polymer (positive or negative).
In the case of uniaxial films, a film having refractive indices such thatnx>ny; nx>nz; and ny˜nz  (3a)
is denoted as a “+A” plate film. In these films, the x direction of the film has a high refractive index, whereas the y and thickness directions are approximately equal in magnitude, and each lower than nx. This type of film is also referred to as a positive uniaxial crystal structure with the optic axis along the x-direction. Such films are easy to make by uniaxially stretching a positively birefringent material, using for example, a film drafter.
In contrast, a “−A” uniaxial film is defined asnx<ny; nx<nz; and ny˜nz  (3b)
where the x-axis refractive index is lower than each of the other directions, which are approximately equal to each other. The most common method for making a −A plate firm is to stretch a negative birefringent polymer, or alternately, to coat a negatively birefringent liquid crystal polymer onto a surface such that the molecules are lined up in a preferred direction (for example, by using an underlying etched orientation layer.)
Another class of uniaxial optical film is the C plate which can also be “+C” or “−C”. The difference between a C and A plate is that in a C plate, the unique refractive index (or optical axis) is in the thickness direction as opposed to in the plane of the film. Thus,nz>ny=nx “+C” plate  (4a)nz<ny=nx “−C” plate  (4b)
C plates can be made by biaxial stretching, holding the relative stretch in the x and y directions constant. Alternately, they can be made by compression forming. Compressing or equibiaxially-stretching an initially isotropic, positive intrinsic birefringent material will result in a −C plate, since the effective orientation direction is in the plane of the film. Conversely, a +C plate is made by compressing or equibiaxially stretching an initially isotropic film made with negative intrinsic birefringent material. In the case of biaxial stretching, if the orientation level is not kept the same in the machine and transvere directions, then the material is no longer a true C plate, but instead is a biaxial film with 2 optical axes.
A third, and more common option for producing C plates takes advantage of the stresses that form during solvent casting of a film. Tensile stresses are created in the plane of the film due to the restraint imposed by the casting belt, which are also equi-biaxial in nature. These stresses tend to align the chains in the plane of the film resulting in −C or +C films, for positive and negative intrinsic birefringent materials respectively. As most cellulose ester films used in displays are solvent cast, and all are essentially positive birefringent, then it is apparent that solvent cast cellulose esters normally produce −C plates. These films can also be uniaxially stretched to produce +A plates (assuming the initial as-cast retardation is very low), but the ability to make +C or −A plates with cellulose esters is extremely limited.
Besides uniaxial plates, it is also possible to use biaxial oriented films to produce C plates. Biaxial films are quantified in a variety of ways including simply listing the 3 refractive indices in the principal directions (along with the direction of these principal axes). Alternately, biaxial films are often quantified in terms of the parameter Nz, where Nz is defined asNz=(nx−nz)/(nx−ny)  (5)
Nz is a measure of the effective out-of-plane birefringence relative to the in-plane birefringence and is typically chosen to be about 0.5 when the film is used as a compensator film for a pair of crossed polarizers.
In order for compensator films to properly eliminate light leakage, they must be combined in certain ways depending on the type of liquid crystal cell in use. For example, Fundamentals of Liquid Crystal Displays (D. K. Yang and S. T. Wu, Wiley, N.J. 2006, pp 208-237), describes various ways to compensate for IPS (in-plane switching), twisted nematic (TN) and VA (vertical alignment) type cells using combinations of uniaxial plates (biaxial plates are also effective but more complicated mathematically). In the case of an IPS cell, a +C plate followed by a +A plate is described (also described is +A followed by +C). When sandwiched between the crossed polars, these films effectively correct for light leakage. Another type of structure is one where an +A plate is used with an −A plate, which gives a more symmetric viewing angle performance than the +A/+C combination.
VA compensated films are similar, although the liquid crystal layer itself acts as a +C structure that has to be figured into the calculation (unlike IPS systems where the cell is typically more “neutral”). The end result is that a +A film in conjunction with a −C film is required. This structure can be made solely with positive birefringent materials. However, improved performance is shown if a 3 layer compensator composed of a +A, −A and −C film, which once again requires a negative birefringence for the −A layer. Note that the structures described above are just a few of many combinations available and are only meant to illustrate the importance of positive and negative birefringence. Other compensators (e.g. biax films and twisted films) are also possibilities that can benefit from having negative intrinsic birefringence.
Although the discussion above identifies benefits of negative intrinsic birefringence, there is also benefit to having a material with a very low or zero intrinsic birefringence. These zero or near zero retardation films have particular importance in IPS type structures as protective layers, such as the above example of an IPS compensator with a +A and a −A film with no C type structures involved. Unfortunately, a typical protective cellulose triacetate (TAC) film that is made by solvent casting and laminated to the polarizer, will have a nominal Rth of from −20 nm to about −70 nm at 633 nm wavelength. Because this film is also in the “optical train” between polarizers, its retardation contributes to the light leakage. Thus, these protective films have effectively added two −C plates to the overall structure (i.e., one for the top polarizer and a second for the bottom polarizer/analyzer) which negates the benefits of the +A/−A compensation strategy. Therefore, in order to alleviate this, protective TAC films having zero Rth (i.e. “Zero-TAC” films) are desirable. Zero-TAC films are also useful as substrates for various coatings including negatively birefringent coatings, and liquid crystal coatings. In these applications, the coating is meant to serve as the compensator and the TAC substrate should be neutral. Such Zero-TAC films are often made by taking cellulose triacetate and incorporating additives that reduce the retardation, but such additives are expensive and alter other performance properties of the film. Having a cellulose ester that has a very low, or zero intrinsic birefringence will allow for the production of Zero-TAC films without the use of significant retardation inhibitors/additives and is one embodiment of the present invention.
As mentioned above, Rth values for solvent cast cellulose triacetate range from about −20 to −70 nm but with mixed ester systems we have observed ranges from about −20 to −300 nm depending on the type of cellulose ester involved (which determines its intrinsic birefringence), the time left on the casting belt (which controls the residual stress in the film), and the type of plasticizers and additives used. Note that by “mixed ester” we are referring to cellulose esters having more than one ester type such as, for example, cellulose acetate propionate (CAP) or cellulose acetate butyrate (CAB). As already described, it is fairly common to add retardation additives or inhibitors to the solvent dope to help raise or lower the as-cast retardation. The amount of retardation in the −C plate can also be enhanced by biaxial stretching or compression or a other films such as a +A plate can be made by subsequent uniaxial stretching (assuming the retardation of the −C film is low initially). Of note, however, is that −A and +C compensator plates cannot be easily made with cellulose esters because of their positive birefringent nature. Thus other more costly (or poorer performing) materials have been used instead.
Melt extruded films also have certain ranges of retardation although the stress profile is different from solvent casting. With melt extrusion, tensile stresses in the MD direction due to drawdown between roll and die coupled with transverse stresses due to the near constant width casting process, result in a film with a biaxial birefringence profile. Nevertheless, the Rth values of these films will always be negative when casting a positive birefringent material. Having a melt processable cellulose ester with negative intrinsic birefringence provides a way to make melt extruded films with positive Rth values (i.e., +C plate behavior). Furthermore, having a zero or negative intrinsic birefringent resin that is both melt and solvent castable provides a very versatile material that can be used on a wider range of film processing equipment.
In summary, there is a need in the art for cellulose esters having very low or negative intrinsic birefringence that are preferably both solvent and melt castable, and maintain good solubility for ease of manufacture. This would allow for greater versatility and performance in the preparation of such as liquid crystal display (LCD) films, such as compensator structures. Such a resin is the basis of the present invention. An LCD film or sheet, as used herein, refers to an optical film or sheet in an LCD assembly, capable, for example, of directing, diffusing, or polarizing light.